1. Field of the Invention
The present invention relates to a servo controlling method for head tracking that can be applicable to a data storage system for use in a computer using a magnetic tape. The present invention also relates to a recording/reproducing apparatus that is capable of executing the foregoing servo controlling method. Further, the invention relates to a magnetic tape in which various types of information for execution of the servo controlling method can be written, and relates to a magnetic tape cartridge. More specifically, the present invention relates to a data storage system for a computer that has excellent servo controlling properties.
2. Description of Related Art
The magnetic tapes can be used in various applications such as audio tapes, video tapes, computer tapes. In the field of tapes for data backup particularly, as the capacity of the hard disk whose contents are to be backed up increases, magnetic tapes having a recording capacity of 800 gigabytes or more per one reel is commercialized. Further, as the technique for backing up data of more than 4 terabytes is to be developed in the future, a large-capacity backup tape has been proposed.
To increase the capacity of a magnetic tape, for example, the following methods are available: a method of reducing the thickness of the magnetic tape so as to increase the tape length per one reel without increasing the reel diameter of the magnetic tape; a method of shortening the recording wavelength of data to be recorded in a magnetic tape so as to increase the recording density in the length direction of a magnetic tape; and, a method of narrowing the recording track width of a magnetic tape so as to increase the recording density in the width direction of the magnetic tape (recording density increasing technique).
As the recording track width is decreased by applying the magnetic tape recording density increasing technique, a magnetic head comes to fail to follow a recording track accurately owing to a position change in the width direction of the magnetic tape upon reproduction of data, which tends to result in an error. Then, presently dominant is a system in which a servo signal is recorded in a magnetic layer or a backcoating layer when a magnetic tape is manufactured and the servo control with use of the foregoing servo signal is carried out (a system using a servo mechanism).
As the servo method, the magnetic servo method and the optical servo method are available. The magnetic servo method is a method in which a servo signal are recorded magnetically in a magnetic layer of a magnetic tape and the servo tracking is carried out by magnetically reading the foregoing servo signal. The optical servo method is a method in which a servo signal composed of recess arrays are formed in a backcoating layer of a magnetic tape by laser irradiation or the like and the servo tracking is carried out by optically reading the recess array.
With such a servo method, when data are recorded in a magnetic tape or when data are reproduced from the magnetic tape, it is possible to enable a magnetic head to follow a recording track even if the position of the magnetic tape is varied relative to the magnetic head in the width direction. More specifically, a servo signal recorded in the magnetic tape is read by the servo head. Next, the position of a head unit (including at least a data-recording head and a data-reproducing head) in the width direction of the magnetic tape is controlled according to the servo signal thus read, so that the data-recording head or data-reproducing head is enabled to follow the recording track. This makes it possible to record information accurately in the magnetic tape, and to accurately reproduce information recorded in the magnetic tape.
As an example of the above-described servo method, the timing-based servo method is disclosed in Patent Document 1 (JP 8(1996)-30942 A) and Patent Document 2 (U.S. Pat. No. 6,226,688). In the timing-based servo method, a servo signal, in patterns tilted with respect to the width direction of the magnetic tape, are recorded in a magnetic tape. With reference to a time interval of peaks of a reproduction waveform obtained when the foregoing servo signal is reproduced, the head position is recognized.
However, the configuration disclosed in the Patent Documents 1 and 2 has the following problem: as the recording track width is narrowed with the increase in the recording density of the magnetic tape, an off-track value increases, which results in an increase in the time until the magnetic head is enabled to follow a recording track.